The present invention relates to acoustic silencers or mufflers and more particularly to a silencer which can efficiently dissipate low frequency acoustic energy. One of the many possible applications is in jet exhaust silencers. Though the invention is described primarily in its embodiment for jet engine test cell exhaust silencer, it is understood that it can be applied as well to inlet and discharge silencers for any kind of noise sources with substantial low frequency sound output such as gas turbines, HVAC and industrial fans, internal combustion engines, etc.
For out-of-airframe testing, jet engines are typically mounted in a test cell and for in-airframe in a Hush House where the jet exhaust is directed through a discharge muffler to reduce the acoustic energy released into the environment. Conventional passive exhaust silencers, however, are deficient at low frequencies where the input impedance of a practically sized absorptive liner would becomes so large that it impedes acoustic flow through the liner.
While active sound cancellation technologies have been developed for various duct environments, e.g., HVAC distribution systems, the conventional approach of generating a cancelling wave is not readily adaptable to the jet engine exhaust problem or to any other application where the flow velocity in the passage is high enough to interfere with the sensing of the approaching sound wave. Firstly, the jet exhaust environment is hostile in the sense that there is substantial heat and turbulence in the exhaust and corrosive gases are present. Similarly hostile environments exist in many other applications such as gas turbine, internal combustion engines and power plant exhaust applications. Accordingly, direct interfacing of sensing and driver transducers to the exhaust duct is not practical. Further, previously known active acoustic attenuation systems accomplish the attenuation of sound by the introduction, into the duct or passage, of a cancelling sound which is ideally a mirror image of the incoming undesirable sound. This cancels the sound downstream of the cancelling sound source and introducing a new sound wave that propagates in the upstream direction. However, if the source of the unwanted sound does not absorb the upstream-propagating sound and there is no effective dissipative section between the noise source and the cancelling driver transducer, the sound energy can build up to a high level in the duct upstream of the cancelling transducer. This results in a large difference in sound pressure across the active silencer section but only a much smaller reduction of the sound propagating downstream.
Among the several objects of the present invention may be noted the provision of novel apparatus for reducing acoustic energy in a duct; the provision of such apparatus which provides improved reduction of acoustic energy at low frequencies as compared with conventional mufflers; the provision of such apparatus which can substantially reduce the low frequency sound energy of jet exhausts, gas turbines, HVAC and industrial fans, internal combustion engines and other sources of noise which contain high intensity low frequency components; the provision of such apparatus which employs an active system to increase dissipation of acoustic energy; the provision of such apparatus where neither the sensors or actuators employed are directly exposed to the flow; the provision of such apparatus which does not generate increased sound energy build up in the upstream portion of a duct; the provision of such apparatus which is highly reliable and which is of relatively simple and inexpensive to construct. Other objects and features will be in part apparent and in part pointed out hereinafter.